Passive radio frequency identification (RFID) tags, as they themselves are not designed with any battery, operate relying on electromagnetic energy sent from a card reader. Due to their simple structure and economical practicality, passive RFID tags have been widely applied in the fields of logistics management, asset tracking and mobile healthcare.
When a passive RFID tag operates, it will absorb electromagnetic energy, sent from a card reader, from the surrounding. After absorbing the energy, the passive RFID tag rectifies part of the energy into DC power for powering internal circuits of the passive RFID tag; and the passive RFID tag further inputs the other part of the energy to an internal modulation/demodulation circuit which will demodulate an amplitude modulation signal carried in this energy and send the demodulated signal to a digital baseband portion of the passive RFID tag for processing.
As the distance between the passive RFID tag and the card reader varies, the electromagnetic energy absorbed by the passive RFID tag during operating from the surrounding varies too. When the passive RFID tag is too close to the card reader or the electromagnetic energy sent from the card reader is too high, the strength of a signal received by the passive RFID tag will also be high, so that the voltage sensed on the coil exceeds the voltage-withstanding limit of a transistor for the rectifier module in the chip. As a result, the transistor is damaged permanently, and the RFID tag no longer functions.
The passive RFID tag transmits data to the card reader in a load modulation manner, and the coil at the card reader side acquires the data upon detecting change in the impedance of the coil at the RFID tag side. When the passive RFID tag is too close to the card reader or the electromagnetic energy sent from the card reader is too high, a load modulation signal coupled from the RFID tag side is likely to result in saturation of the receiving end of the card reader, thus to fail the communication. Such failure is more likely to occur in an RTF (Reader Talk First) communication mode where the card reader sends a command first and then waits for a response from the RFID tag.
In order to solve the aforementioned problems of voltage-withstanding reliability and reception saturation of the card reader, it is required to provide an amplitude limitation processing circuit in the interior of an RFID tag chip circuit, in order to ensure that the voltage across both ends of an antenna on the RFID tag is limited to a predetermined value. The implementation of amplitude limitation may be carried out by a method of leaking current from a rectifier branch to the ground, so that the voltage level output by a rectifier is controlled. The most ideal design requires that a current leakage path may be effectively cut off in the case of an extremely weak field, that is, the current is completely not leaked; and in the case of a gradually enhanced field, the current leakage path may control the starting point of current leakage and the amount of leakage current at any time, so as to achieve the purpose of dynamic adjustment.
In the Invention Patent Application No. 201010568305.4, filed on Nov. 30, 2010 by the University of Electronic Science and Technology and entitled Demodulator Circuit for Ultrahigh-frequency Radio Frequency Identification Tap Chip, the amplitude limiting function is realized by a voltage comparator. The voltage comparator compares a magnitude relationship between envelope signals (i.e., data information) generated by an envelope detection circuit and a means value of the envelop signals themselves, so that an inverter following the voltage comparator outputs high or low logic signals, i.e., demodulated signals. As such, even in the case where the signal intensity difference is very large due to a too small distance or a too large distance from a card reader to an electronic tag, the demodulator circuit is still able to self-adaptively ensure the stability of a mean value generation circuit, so as to ensure the normal and stable operation of the demodulator circuit.
The technology of self-adaptively adjusting voltage at the gate of a first PMOS transistor in the above patent is a method for indirectly monitoring the intensity of radio frequency signals. The object of the dynamic adjustment is a PMOS transistor serving as an equivalent resistor, so that the equivalent resistance of the channels of the PMOS transistor under different signal intensities remains stable, that is, the time constant of the filter formed together with a capacitor remains stable, thereby finally realizing stable detection effects and ensuring the normal and stable operation of the demodulator circuit. In the present application, the intensity of radio frequency signals is monitored by the dynamic adjustment of the voltage amplitude of the rectifier, and the object of adjustment is a limiter circuit of the rectifier, so that a current leakage path from an antenna port to the ground is opened to different degrees under different signal intensities, thereby protecting transistors at the radio frequency front-end from being influenced by a high voltage and avoiding saturation of reception at the card reader side.
The PCT Patent Application No. 200880129721.5, filed on Jun. 9, 2008 by Shimadzu Corporation and entitled Limiter Circuit, disclosed a limiter circuit for cutting off voltage values exceeding upper and lower threshold signals by switching on or off transistors. This limiter circuit is used for limiting the signal intensity and is specific to circuits having a constant power input. This limiter circuit is different from the rectifier control circuit used in the present application, and also unable to be applied in an RFID circuit.
In the Invention Patent Application No. 200710058875.7, filed on Aug. 20, 2007 by Tianjin Nandaqiangxin Semiconductor Chip Design Co., LTD. and entitled Radio Frequency Identification Tag Circuit System Structure and Operating Methods and Applications thereof, the unique inventive point is that a power supply line from a rectifier is divided into several paths for different modules, and a viewpoint of improving energy conversion and utilization efficiency is thus provided. First, dividing a power supply line into several paths to be connected to different modules is a common practice in the chip design, and this application fails to clearly explain how to improve the energy conversion and utilization efficiency and what extent the energy conversion and utilization efficiency are improved. If it is desirable to really realize efficiency improvement, the connection method mentioned in this application (it itself is a common connection method) is not enough, and directly supplying power to a high voltage generation circuit of a storage control module by a rectifier output branch will even result in very high power consumption of an oscillator used in a charge pump. Second, this application does not involve the aspect of adjusting the voltage amplitude at the input terminal of a rectifier by switching signals in the present patent application, and thus is not overlapped with the inventive point of the present patent application.
In the Invention Patent No. 200610024814.4, filed by Shanghai Huahong Integrated Circuit Co., LTD. and entitled Limiter and Protector Circuit for Non-contact IC card and Radio Frequency Tag Chip, the following concept was proposed: in a strong field, energy obtained by antenna coupling is limited within an acceptable range, without influencing the normal operation of the subsequent demodulator circuit, voltage stabilizer circuit, docking circuit and reset circuit inside the chip. The differences between this patent and the present application are as follows.
1. Due to different purposes of the limiter circuits, the limiter circuit provided by Shanghai Honghua generates amplitude limiting signals by controlling the charging or discharging of a capacitor by a high-voltage or low-voltage detection circuit. The change in specific field intensity of the amplitude limiting signals has an obviously large time constant, that is, the change in voltage amplitude on the antenna is slowly reflected. This technology can not achieve the purpose of over-voltage protection as provided in the present application because the transistors inside the radio frequency tag chip have been in an unreliable over-voltage driven state within the limited time constant. Therefore, the limiter circuit disclosed by the present application has a very quick response speed to the change in voltage on the antenna and thus may provide for good protection.
2. The limiter circuit provided by Shanghai Honghua has two discharge paths. One discharge path is a slow path, as described above, which is not suitable to over-voltage protection, while the other one is a discharge path controlled by demodulation signals. This technology is substantially different from the technology disclosed by the present application. Among many low-cost radio frequency tag chips applied in access control cards and other fields, a card reader just provides field energy, and the operation of a tag circuit is limited to a simple mode where “the tag is activated and powered after being coupled to energy, and then directly sends information stored in the tag”. Such cards has no demodulator circuit as used in this patent and are thus unable to provide signals for controlling the discharge paths. In the limiter circuit disclosed by the present application, the path of the limiter circuit is controlled by the power supply voltage and switching signals output by a judgment circuit, and this is nothing to do with the presence or absence of a demodulator circuit and thus the characteristics of the demodulation signals themselves output by the demodulator circuit. In contrast, the technology disclosed by the present application has a broader range of application in both high-performance radio frequency tags and low-cost radio frequency tags.
3. The switching transistor controlled by high-voltage detection and low-detection signals in Shanghai Honghua Patent has two constant current sources as bias. This will result in large DC power consumption in a passive radio frequency tag system and is disadvantageous for the realization of low power consumption and high sensitivity. The technology disclosed by the present application controls a number of paths having different discharge capabilities to be opened and closed by switching signals output by power detection and judgment circuits, so as to achieve the purpose of amplitude limitation. The technology disclosed by the present application details discharge of current and has better energy efficiency.
4. The limiter circuit provided by Shanghai Honghua has two judgment points, i.e., a critical point for a too low voltage and a critical point for a too high voltage, for detecting the voltage across two ends of an antenna. When the voltage across two ends of the antenna is lower than the critical point for a too low voltage, the charge on a capacitor is discharged. When the voltage across two ends of the antenna is higher than the critical point for a too high voltage, the charge on a capacitor is charged and accumulated. The problem is that two control switches, i.e., an upper control switch and a lower control switch, are both in an OFF state when the voltage across two ends of the antenna is between the two critical points, and the voltage on the capacitor is floating and independent of control by any signal. This is a fatal problem in a passive radio frequency tag chip, and is likely to result in uncontrollable current discharge and thus energy loss, thereby affecting the sensitivity of the tag.